Nice hotel. I stayed there a couple of years ago when the price was ridiculously low. Location is okay. Just to the south of Namba station so you can walk to Dotonbori. Namba is on the south side of central Osaka which puts it closer to KIX than most other places that you might consider.
With a 6:30 AM departure I guess you’re planning to use a car service. You’ll need to leave your hotel in the wee hours whether you leave from Kyoto or Osaka. Traffic is not a major concern at that time in the morning, although I guess there’s a chance of encountering overnight roadworks in places.
Moving to the Fairfield in Namba probably gives you an extra 30 minutes in bed. Or to think about it another way, that’s 30 minutes less time in the car with irritable kids.